An external filter would be necessary, as you'd never fit one inside.
Then again, I'm wondering if the bioload of the shrimp would even be big enough that you'd need a filter, especially if you're putting plants in it as well.
I had the same thought
It would have an inch of gravel and then an air pump under the gravel
This could work as some sort of filter
I would also put some hardy plants in there and maybe a moss ball
a 2 liter is too small for any living organisms. Even shrimp don't deserve such limited swimming space. Even SPIDERS don't deserve to live in a blender ( unless it's turned on LOL ).